As one of the methods for synthesizing liquid fuels from a natural gas, a GTL (Gas To Liquids: a liquid fuel synthesis) technique has recently been developed. In the GTL technique, a natural gas is reformed to synthesize a synthesis gas containing a carbon monoxide gas (CO) and a hydrogen gas (H2) as main components, and then, hydrocarbon compounds are synthesized by the Fischer-Tropsch synthesis reaction using the synthesis gas as a feedstock gas. Further, in the GTL technique, the hydrocarbon compounds are hydrogenated and fractionally distilled to produce liquid fuel products, such as a naphtha (raw gasoline), a kerosene, a gas oil, and a wax.
Since the liquid fuel products from the hydrocarbon compounds used as a feedstock have high paraffin content, and include no sulfur components, for example, as shown in Patent Document 1, the liquid fuel products attract attention as environment-friendly fuels.
In a synthesis reactor which performs the Fisher-Tropsch synthesis reaction, heavy hydrocarbon compounds with a comparatively high number of carbon atoms are produced as a liquid, and light hydrocarbon compounds with a comparatively low number of carbon atoms (mainly including hydrocarbons equivalent to naphtha) are generated as a gas.
As an example of a method for obtaining liquid-fuel products from the light and heavy hydrocarbon compounds, the following process is mentioned. First, the light hydrocarbon compounds discharged as a gas from the synthesis reactor are cooled down and liquefied by a heat exchanger. And the liquefied light hydrocarbon compounds are separated and recovered in a gas-liquid separator. Then, the recovered light hydrocarbon compounds are mixed with the heavy hydrocarbon compounds discharged as a liquid from the synthesis reactor, and are brought to a fractionator.
Then, the hydrocarbon compounds are fractionally distilled according to boiling points in the fractionator, and are fractionally distilled into a naphtha fraction (the boiling point of which is lower than about 150° C.), a middle distillate equivalent to a kerosene and a gas oil (the boiling point of which is about 150 to 360° C.), and a wax fraction (the boiling point of which is higher than about 360° C.).
The naphtha fraction, the middle distillate, and the wax fraction are hydrotreated respectively to produce liquid fuels and other products, such as a naphtha, a kerosene, a gas oil, or a wax.